1. Field of the Invention
The present invention relates to a ring transmission system of the bi-directional line switched ring type and to a squelch method of the system.
Systems of a plurality of nodes connected by ring transmission lines can be roughly classified into the uni-directional path switched ring (UPSR) type and the bi-directional line switched ring (BLSR) type.
Compared with the former uni-directional path switched ring type, the latter bi-directional line switched ring type enables use of the same channel among different nodes and thus has the advantage that the line capacity can be made larger. In this bi-directional line switched ring type, when a failure occurs at a number of positions and the ring transmission line is disconnected, a signal which cannot reach its destination node is generated. That signal is sometimes transmitted to another node by the loopback function used so as to repair the failure. In this case, erroneous inter-node communication would occur, therefore a so-called "squelch" operation in which the signal which cannot reach its destination node is replaced by a path alarm indication signal (P-AIS) which is transmitted instead is carried out. There is a demand for this squelch processing to be carried out with a good efficiency and at a high speed.
2. Description of the Related Art
As will be explained in detail later by using the drawings, a squelch table is formed at each node in correspondence with the channels using the ID numbers of the transmission nodes (nodes to which the signals are added) and the ID numbers of the reception nodes (nodes on which the signals are dropped) at the time of setting up the communication channels among the nodes. When all of the control for insertion of a squelch operation using this squelch table when a number of failures occur is carried out by software, the processing for searching through the squelch table based on the ID numbers for the nodes not able to receive signals is carried out in correspondence with the channels by a processor. In this case, the collation and comparison of the ID numbers are carried out one after the other, therefore there is the problem that a long time is required for a squelch decision.
Further, in a bi-directional line switched ring (BLSR) type ring transmission system comprised of optical fiber transmission lines of a length of 1200 km and 16 nodes, it is desired that the time from the detection of a failure to the loopback for repairing the failure and the completion of the switch between the working and protection lines be not more than 50 ms. Further, when a plurality of failures occur, it has been desired that the time be not more than 100 ms. Accordingly, it becomes necessary to carry out the switching at a high speed after the detection of a failure. Further, in the case of a plurality of failures, squelch processing becomes necessary, so this squelch processing must also be carried out at a higher speed.
Therefore, consideration may be given to a configuration which enables all of the squelch processing to be all carried out by hardware. However, since a squelch table stores the node ID numbers in correspondence with the channels and in correspondence with the transmission or reception direction of the signals, there is a problem that the circuit configuration for collating and comparing the squelch table and the ID numbers of the nodes which signals cannot reach due to a plurality of failures becomes very complex and large in scale, thus realization is difficult.